The present invention relates to supports for silver catalysts used in the vapor phase epoxidation of olefins and more particularly for the production of ethylene oxide from ethylene and molecular oxygen.
The production of ethylene oxide is carried out in the vapor phase in fixed bed tubular reactors by reacting oxygen and ethylene over silver catalytic phases which are deposited onto refractory and inert supports. These supports are made mainly of alumina, silica-alumina, magnesia, pumice stone, zirconia, clay, ceramics, natural or artificial graphite, asbestos, natural or artificial zeolite, or silicon carbide. Prior art shows a preference for solids with low specific surface areas. All the claimed supports have surface areas of less than 10 sq. m/g. Thus, in French Pat. No. 2,253,747, a whole series of porous products are mentioned, namely: silicon carbide, zirconia, silica, silica-alumina, .alpha.-alumina, with surface areas of between 0.04 and 10 sq. m/g. Specific surface areas in the same region: 0.03 to 10 sq. m/g, are also claimed in French Pat. Nos. 2,117,183; 2,130,465; 2,167,728; 2,249,087; and 2,271,869 using commercially available alumina or silica-alumina based supports. In fact, the specific areas of the supports employed are distinctly lower and figures of less than 1 sq. m/g are claimed in French Pat. Nos. 2,243,193 (alumina, silicon carbide, silica-alumina) and 1,522,279 (silica-alumina). These specific surface areas are determined in the majority of cases by the method of nitrogen adsorption, known as the B.E.T. method, described by BRUNAUER, EMMET and TELLER in "The Journal of the American Chemical Society", vol. 60, page 309, 1938. The second important property of these supports is their porosity. There is agreement in the prior art that a porosity which may reach 60 percent by volume is favorable to the activity and selectivity for ethylene oxide of the catalysts obtained. On the other hand, a certain divergence exists in the claims regarding the dimensions of the pores. The extent or degree of divergence is shown distinctly by the following figures:
I. 0.2 to 0.4 microns in French Pat. No. 2,249,087 PA1 II. 1 to 15 microns in French Pat. Nos. 2,253,747; 2,117,183 and in Belgian Pat. No. 848,659 for alumina or silica-alumina supports with low particle size PA1 III. 50 to 200 microns in French Pat. Nos. 1,354,391 and 1,413,213 for alumina or silicon carbide supports having a large particle size which may reach as much as 9 mm PA1 IV. 10 to 300 microns in French Pat. Nos. 2,243,193; 2,023,984; and 2,208,713 and in British Pat. No. 1,133,484 PA1 V. 50 to 1,500 microns in French Pat. Nos. 2,029,751; 2,059,124 and Czech Pat. No. 130,654, for supports on the basis of alumina with a high particle size which may be as much as 9 mm. PA1 (a) impregnation of the support with a solution of the chosen silver compound. This impregnation may be carried out either by immersion of the support in the solution, or by the continuous sprinkling of the support under reduced pressure and at a temperature which permits the immediate elimination of the solvent. PA1 (b) drying of the impregnated support. PA1 (c) thermal treatment of the product obtained so as to liberate the silver from the compound deposited on the support.
The importance of the relationship between the diameter of the pores and the particle size of the catalysts for the synthesis of ethylene oxide has been pointed out in two patents: French Pat. No. 2,253,747, which points out that the increase in the particle size of the silica-alumina supports must be accompanied by an increase in the mean diameter of the pores, and French patent application No. 77/21118, which claims, in the case of artificial graphite supports, the beneficial effect of an increase in the diameter of the macropores with the particle size.